The present invention relates generally to servo systems in disc drives. In particular, the present invention relates to compensation for errors in servo systems.
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo sub-system to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high-frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable run-out errors because they cause the same errors each time the head passes along a track. As track densities increase, these repeatable run-out errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the servo fields can result in a track interfering with or squeezing an adjacent track. This is especially acute when a first written-in error causes a head to be outside of an inner track""s ideal circular path and a second written-in error causes the head to be inside of an outer track""s ideal circular path. To avoid limitations on the track pitch, systems that compensate for repeatable run-out errors are employed.
One existing technique for repeatable run-out error compensation involves obtaining a sequence of repeatable run-out values, computing compensation values based on the repeatable run-out values, and storing the compensation values in compensation tables. These compensation values are then injected into the servo loop to compensate for repeatable run-out errors. In this technique, the sequence of repeatable run-out errors is obtained by repeatedly following tracks on the discs over a number of revolutions and averaging the position error signals obtained at each servo field over all of the revolutions. This averaging process is time consuming and complex. In addition, the repeatable run-out compensation values cannot be obtained in real-time , during disc operation, by using this technique.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.
The present embodiments relate to disc drive servo systems that employ a real-time adaptive repeatable run-out compensation scheme to compensate for written-in repeatable run-out errors in the servo system, thereby addressing the above-mentioned problems.
An apparatus and method of correcting for written-in repeatable run-out in a disc drive having a servo loop for positioning a head over a rotating disc is provided. The rotating disc has at least one data track and servo information recorded in a plurality of servo fields along the data track. An initial written-in repeatable run-out compensation value for each servo field is computed as a function of a position error signal generated for each servo field during a first revolution of the disc. The initial written-in repeatable run-out compensation value for each servo field is then injected into the servo loop during another revolution of the disc. A compensated position error signal for each servo field is computed as a function of the initial written-in repeatable run-out compensation value for each servo field. A refined written-in repeatable run-out compensation value for each servo field is then computed as a function of the compensated position error signal for each servo field.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.